(And they say) i.e. the Jews: (Our hearts are hardened) vis-à-vis what you say, O Muhammad, i.e. our hearts are capable of absorbing any knowledge, but they prove incapable of comprehending your knowledge or what you say. (Nay) this is a reply to them, (but Allah hath cursed them) sealed their hearts (for their disbelief) as a punishment for their disbelief. (Little is that which they believe) they believe neither much nor a little; it is also said that this means: they do not believe in what you say at all.
